A ‘new’ microsporidian in the UK: investigations into the unknown microsporidian infecting juvenile smelt (Osmerus eperlanus) from the River Thames

Abstract

The European smelt Osmerus eperlanus is a small anadromous species, which resides in coastal waters with a broad distribution around Europe and the Baltic regions. UK populations were once abundant, inhabiting most rivers around the UK. However, pollution and overexploitation of this once commercial species led to its disappearance from most UK rivers during the 1800s. With the improved water quality and legal protection, the species returned to many rivers, including a breeding population in the River Thames since the 1980s. This species is severely understudied with knowledge gaps on life-history traits, dynamics between different populations in the UK and native parasitic fauna. In the summer of 2018, Environment Agency (EA) surveys in the Thames revealed that many juvenile smelt were infected with a microsporidian parasite, previously unreported in the UK smelt. This doctoral thesis aimed to identify and describe this newly reported microsporidian and assess the possible impacts of infection. Using traditional histological analysis and micro-computer tomography (micro-CT), I characterised the parasite as a species of the Glugea genus. The micro-CT imagery provided a novel approach for assessing a full 360° view of parasitic infections, allowing for visualisation of the distribution of disease and providing quantitative measures for the volume of cysts within the body cavity. Additional phylogenetic analysis confirmed the Glugea genus classification and revealed close sequence identity to several other Glugea species. Based on the morphological presentation, phylogenetic placement, and historical findings of a morphologically similar microsporidian infecting smelt from the Baltic regions and Rainbow smelt Osmerus mordax in the US and Canada; I suggest that the microsporidian infecting Thames smelt is likely G. hertwigi. This study was the first to provide a ribosomal DNA sequence for a microsporidian of the Glugea genus in European smelt and the first to report its presence in UK smelt. Furthermore, I assessed the surface pressure changes and drag forces associated with the disease due to the gross distortions caused to the fish's body by bulging cysts. To the best of my knowledge, this study is the first to attempt to apply computational fluid dynamics to calculate costs associated with parasitic infection in fish. The data presented in this thesis adds to the limited body of knowledge on smelt populations in the UK and provides novel complementary methods for assessing parasitic infections in fish.